1. Field of the Invention
The present invention concerns an amplifier circuit for hearing aids of the type having an amplification device with an input at which acoustically-generated signals and inductively-generated signals are present, and a filter device that is connected to the amplification device for compensation of an acoustic feedback by adaptation to the acoustic feedback. In addition, the present invention concerns a hearing aid with such an amplifier circuit. Furthermore, the present invention concerns a corresponding method for the amplification of input signals of a hearing aid.
2. Description of the Prior Art
In a hearing aid, acoustic input signals are acquired by a microphone, amplified in an amplification device, and the amplified signals are supplied to a speaker (earphone). The sound emitted from the speaker is partially fed back to the microphone, resulting in noticeable interference.
Besides the microphone and the speaker, a hearing aid often also has a telecoil for the inductive infeed of auditory signals, for example from a telephone. Since the speaker operates electromagnetically, it normally causes a magnetic feedback at the telecoil from the magnet of the speaker.
In mixed mode, the microphone and the telecoil are simultaneously operated, and a number of input signals exist. Correspondingly, a number of feedback paths are also created in the mixed mode.
In this context, a device for electromagnetically reducing feedback in communications devices is known from the European Application EP 1 119 218 A1. A digital filter is used in order to simulate the combined pulse response of the components of the D/A converter, speaker, electromagnetic feedback path, pick-up coil, and A/D converter. The coefficients of the digital filter are adjusted or set according to a selected feedback estimation algorithm.